Normally it is not a good idea to mess around with SQL queries, but as this task is quite simple, you might want to give it a try.
The code from the 
ProfileBannerService:
	
	
	
		PHP:
	
	
		    public function deleteBanner()
    {
        $this->deleteBannerFiles();
        $profile = $this->user->Profile;
        $profile->bulkSet([
            'banner_date' => 0,
            'banner_optimized' => false,
            'banner_position_y' => null,
        ]);
        $profile->save();
        if ($this->logIp)
        {
            $ip = ($this->logIp === true ? $this->app->request()->getIp() : $this->logIp);
            $this->writeIpLog('delete', $ip);
        }
        return true;
    }
    // removed..
    protected function deleteBannerFiles()
    {
        if ($this->user->Profile->banner_date)
        {
            foreach ($this->sizeMap AS $code => $size)
            {
                File::deleteFromAbstractedPath($this->user->Profile->getAbstractedBannerPath($code));
            }
        }
    }
	 
 
So, if you want to remove 
all profile banners at once:
Step 1:
	
	
	
		SQL:
	
	
		UPDATE
    `xf_user_profile`
SET
    `banner_date` = 0,
    `banner_position_y` = NULL,
    `banner_optimized` = 0
	 
 
Step 2:
Remove all profile banners from the web server (they should be in the folders 
data/profile_banners/l and 
data/profile_banners/m).
Notes:
- This will not update the user change log.
- Untested and on your own risk.
- Make a backup before you do this.